path: root/JS.md

# JavaScript API

## Note: This document is no longer being updated. Please see the [normative documentation](http://webassembly.github.io/spec/js-api/index.html).

In the [MVP](MVP.md), the only way to access WebAssembly on the Web is through
an explicit JS API which is defined below.
(In the [future :unicorn:][future general], WebAssembly may also
be loaded and run directly from an HTML `<script type='module'>` tag—and
any other Web API that loads ES6 modules via URL—as part of 
[ES6 Module integration](Modules.md#integration-with-es6-modules).)

WebAssembly JS API declaration file for TypeScript can be found [here](https://github.com/01alchemist/webassembly-types/blob/master/webassembly.d.ts) which enable autocompletion and make TypeScript compiler happy.

## Traps

Whenever WebAssembly semantics specify a [trap](Semantics.md#traps),
a `WebAssembly.RuntimeError` object is thrown. WebAssembly code (currently)
has no way to catch this exception and thus the exception will necessarily
propagate to the enclosing non-WebAssembly caller (either the browser or
JavaScript) where it is handled like a normal JavaScript exception.

If WebAssembly calls JavaScript via import and the JavaScript throws an
exception, the exception is propagated through the WebAssembly activation to the
enclosing caller.

Because JavaScript exceptions can be handled, and JavaScript can continue to
call WebAssembly exports after a trap has been handled, traps do not, in
general, prevent future execution.

## Stack Overflow

Whenever a [stack overflow](Semantics.md#stack-overflow) is happening in
WebAssembly code, the same exception is thrown as for a stack overflow in
JavaScript.

## Out of Memory

Whenever validation, compilation or instantiation run out of memory, the
same exception is thrown as for out of memory conditions in JavaScript.

## The `WebAssembly` object

The `WebAssembly` object is the initial value of the `WebAssembly` property of
the global object. Like the `Math` and `JSON` objects, the `WebAssembly` object
is a plain JS object (not a constructor or function) that acts like a namespace
and has the following properties:

### `WebAssembly [ @@toStringTag ]` Property

The initial value of the [`@@toStringTag`](https://tc39.github.io/ecma262/#sec-well-known-symbols)
property is the String value `"WebAssembly"`.

This property has the attributes { [[Writable]]: `false`, [[Enumerable]]: `false`, [[Configurable]]: `true` }.

### Constructor Properties of the `WebAssembly` object

The following intrinsic objects are added:

* `WebAssembly.Module` : the [`WebAssembly.Module` constructor](#webassemblymodule-constructor)
* `WebAssembly.Instance` : the [`WebAssembly.Instance` constructor](#webassemblyinstance-constructor)
* `WebAssembly.Memory` : the [`WebAssembly.Memory` constructor](#webassemblymemory-constructor)
* `WebAssembly.Table` : the [`WebAssembly.Table` constructor](#webassemblytable-constructor)
* `WebAssembly.CompileError` : a [NativeError](https://tc39.github.io/ecma262/#sec-nativeerror-object-structure)
   which indicates an error during WebAssembly decoding or validation
* `WebAssembly.LinkError` : a [NativeError](https://tc39.github.io/ecma262/#sec-nativeerror-object-structure)
   which indicates an error during WebAssembly instantiating a module (other than traps from the start function)
* `WebAssembly.RuntimeError` : a [NativeError](https://tc39.github.io/ecma262/#sec-nativeerror-object-structure)
   which is thrown whenever WebAssembly specifies a [trap](#traps).

### Function Properties of the `WebAssembly` object

#### `WebAssembly.validate`

The `validate` function has the signature:

```
Boolean validate(BufferSource bytes)
```

If the given `bytes` argument is not a
[`BufferSource`](https://heycam.github.io/webidl/#common-BufferSource),
then a `TypeError` is thrown.

Otherwise, this function performs *validation* as defined by the [WebAssembly
specification](https://github.com/WebAssembly/spec/blob/master/interpreter/) and returns `true` if validation succeeded, `false` if validation failed.

#### `WebAssembly.compile`

The `compile` function has the signature:

```
Promise<WebAssembly.Module> compile(BufferSource bytes)
```

If the given `bytes` argument is not a
[`BufferSource`](https://heycam.github.io/webidl/#common-BufferSource),
the returned `Promise` is [rejected](https://tc39.github.io/ecma262/#sec-rejectpromise)
with a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror).

Otherwise, this function starts an asynchronous task to compile a `WebAssembly.Module`
as described in the [`WebAssembly.Module` constructor](#webassemblymodule-constructor).
On success, the `Promise` is [fulfilled](https://tc39.github.io/ecma262/#sec-fulfillpromise)
with the resulting `WebAssembly.Module` object. On failure, the `Promise` is 
[rejected](https://tc39.github.io/ecma262/#sec-rejectpromise) with a 
`WebAssembly.CompileError`.

The asynchronous compilation is logically performed on a copy of the state of
the given `BufferSource` captured during the call to `compile`; subsequent mutations
of the `BufferSource` after `compile` return do not affect ongoing compilations.

#### `WebAssembly.instantiate`

The `instantiate` function is overloaded based on types of its arguments.
If neither of the following overloads match, then the returned `Promise` is
[rejected](https://tc39.github.io/ecma262/#sec-rejectpromise)
with a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror).

```
dictionary WebAssemblyInstantiatedSource {
   required WebAssembly.Module module;
   required WebAssembly.Instance instance;
};

Promise<WebAssemblyInstantiatedSource>
  instantiate(BufferSource bytes [, importObject])
```

If the given `bytes` argument is not a
[`BufferSource`](https://heycam.github.io/webidl/#common-BufferSource),
the returned `Promise` is [rejected](https://tc39.github.io/ecma262/#sec-rejectpromise)
with a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror).

This function starts an asynchronous task that first compiles a `WebAssembly.Module`
from `bytes` as described in the [`WebAssembly.Module` constructor](#webassemblymodule-constructor)
and then queue a task to instantiate the resulting `Module` with `importObject` as described in the
[`WebAssembly.Instance` constructor](#webassemblyinstance-constructor). After the instantiation task runs
and before any subsequent steps are taken, other unspecified asynchronous tasks may be run.
On success, the `Promise` is [fulfilled](https://tc39.github.io/ecma262/#sec-fulfillpromise)
with a plain JavaScript object pair `{module, instance}` containing the resulting
`WebAssembly.Module` and `WebAssembly.Instance`. The 2 properties `module` and `instance` of the returned pair are  configurable, enumerable and writable. 

On failure, the `Promise` is
[rejected](https://tc39.github.io/ecma262/#sec-rejectpromise) with a 
`WebAssembly.CompileError`, `WebAssembly.LinkError`, or `WebAssembly.RuntimeError`, depending on the cause of failure.

The asynchronous compilation is logically performed on a copy of the state of
the given `BufferSource` captured during the call to `instantiate`; subsequent mutations
of the `BufferSource` after `instantiate` return do not affect ongoing compilations.

```
Promise<WebAssembly.Instance> instantiate(moduleObject [, importObject])
```

This description applies if the first argument is a `WebAssembly.Module` instance.

This function asynchronously queues a task that instantiates a `WebAssembly.Instance`
from `moduleObject` and `importObject` as described in the
[`WebAssembly.Instance` constructor](#webassemblyinstance-constructor). After the instantiation task runs
and before any subsequent steps are taken, other unspecified asynchronous tasks may be run.
On success, the `Promise` is [fulfilled](https://tc39.github.io/ecma262/#sec-fulfillpromise)
with the resulting `WebAssembly.Instance` object. On failure, the `Promise` is
[rejected](https://tc39.github.io/ecma262/#sec-rejectpromise) with a 
`WebAssembly.CompileError`, `WebAssembly.LinkError`, or `WebAssembly.RuntimeError`, depending on the cause of failure.

## `WebAssembly.Module` Objects

A `WebAssembly.Module` object represents the stateless result of compiling a
WebAssembly binary-format module and contains one internal slot:

 * [[Module]] : an [`Ast.module`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L176)
   which is the spec definition of a module

### `WebAssembly.Module` Constructor

The `WebAssembly.Module` constructor has the signature:

```
new Module(BufferSource bytes)
```

If the NewTarget is `undefined`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
exception is thrown (i.e., this constructor cannot be called as a function without `new`).

If the given `bytes` argument is not a
[`BufferSource`](https://heycam.github.io/webidl/#common-BufferSource),
a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
exception is thrown.

Otherwise, this function performs synchronous compilation of the `BufferSource`:

1. The byte range delimited by the `BufferSource` is first logically decoded 
   according to [BinaryEncoding.md](BinaryEncoding.md) and then validated
   according to the rules in [spec/valid.ml](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/valid.ml#L415).
1. The spec `string` values inside `Ast.module` are decoded as UTF8 as described in 
   [Web.md](Web.md#names).
1. On success, a new `WebAssembly.Module` object is returned with [[Module]] set to
   the validated `Ast.module`.
1. On failure, a new `WebAssembly.CompileError` is thrown.

### `WebAssembly.Module.prototype [ @@toStringTag ]` Property

The initial value of the [`@@toStringTag`](https://tc39.github.io/ecma262/#sec-well-known-symbols)
property is the String value `"WebAssembly.Module"`.

This property has the attributes { [[Writable]]: `false`, [[Enumerable]]: `false`, [[Configurable]]: `true` }.

### `WebAssembly.Module.exports`

The `exports` function has the signature:

```
Array exports(moduleObject)
```

If `moduleObject` is not a `WebAssembly.Module`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

This function returns a new `Array` every time it is called. Each such `Array` is produced by mapping each
[`Ast.export`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L152)
`e` of [moduleObject.[[Module]].exports](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L187)
to the Object `{ name: String(e.name), kind: e.ekind }` where `e.name` is [decoded as UTF8](Web.md#names)
and `e.ekind` is mapped to one of the String values `"function"`, `"table"`, `"memory"`, `"global"`.

Note: other fields like `signature` may be added in the future.

The returned `Array` is populated in the same order exports appear in the WebAssembly binary's exports table.

### `WebAssembly.Module.imports`

The `imports` function has the signature:

```
Array imports(moduleObject)
```

If `moduleObject` is not a `WebAssembly.Module`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

This function returns a new `Array` every time it is called. Each such `Array` is produced by mapping each
[`Ast.import`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L167)
`i` of [moduleObject.[[Module]].imports](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L203)
to the Object `{ module: String(i.module_name), name: String(i.item_name), kind: i.ikind }` where
`i.module_name` and `i.item_name` are  [decoded as UTF8](Web.md#names) and
`i.ikind` is mapped to one of the String values `"function"`, `"table"`, `"memory"`, `"global"`.

Note: other fields like `signature` may be added in the future.

The returned `Array` is populated in the same order imports appear in the WebAssembly binary's imports table.

### `WebAssembly.Module.customSections`

The `customSections` function has the signature:

```
Array customSections(moduleObject, sectionName)
```

If `moduleObject` is not a `WebAssembly.Module`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Let `sectionNameString` be the result of [`ToString`](https://tc39.github.io/ecma262/#sec-tostring)(`sectionName`).

This function returns a new `Array` every time it is called. Each such `Array` is produced by mapping each
[custom section](BinaryEncoding.md#high-level-structure) (i.e., section with
`id` 0) whose `name` field ([decoded as UTF-8](Web.md#names)) is equal to
`sectionNameString` to an `ArrayBuffer` containing a copy of the section's
`payload_data`. (Note: `payload_data` does not include `name` or `name_len`.).

The `Array` is populated in the same order custom sections appear in the WebAssembly binary.

### Structured Clone of a `WebAssembly.Module`

A `WebAssembly.Module` is a
[cloneable object](https://html.spec.whatwg.org/multipage/infrastructure.html#cloneable-objects)
which means it can be cloned between windows/workers and also
stored/retrieved into/from an [IDBObjectStore](https://w3c.github.io/IndexedDB/#object-store).
The semantics of a structured clone is as-if the binary source, from which the
`WebAssembly.Module` was compiled, were cloned and recompiled into the target realm.
Engines should attempt to share/reuse internal compiled code when performing
a structured clone although, in corner cases like CPU upgrade or browser
update, this may not be possible and full recompilation may be necessary.

Given the above engine optimizations, structured cloning provides developers
explicit control over both compiled-code caching and cross-window/worker code
sharing.

## `WebAssembly.Instance` Objects

A `WebAssembly.Instance` object represents the instantiation of a 
`WebAssembly.Module` into a
[realm](https://tc39.github.io/ecma262/#sec-code-realms) and has one
internal slot:

* [[Instance]] : an [`Instance.instance`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L17)
  which is the WebAssembly spec definition of an instance
 * [[Exports]] : the exports object created during instantiation

### `WebAssembly.Instance` Constructor

The `WebAssembly.Instance` constructor has the signature:

```
new Instance(moduleObject [, importObject])
```

If the NewTarget is `undefined`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
exception is thrown (i.e., this
constructor cannot be called as a function without `new`).

If `moduleObject` is not a `WebAssembly.Module`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Let `module` be the [`Ast.module`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L176)
`moduleObject.[[Module]]`.

If the `importObject` parameter is not `undefined` and `Type(importObject)` is
not Object, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown. If the list of 
[`module.imports`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L186)
is not empty and `Type(importObject)` is not Object, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Note: Imported JavaScript functions are wrapped as [host function](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L9) values in the following algorithm. For the purpose of the algorithm, a _new_ [host function](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L9) value is always generated fresh and considered distinct from any other previously created host function value, including those wrapping the same JavaScript function object.
Consequently, two [closure](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L7) values are considered equal if and only if:

* Either they are both WebAssembly functions for the same instance and referring to the same function definition.
* Or they are the same host function value.

Let `funcs`, `memories` and `tables` be initially-empty lists of callable JavaScript objects, `WebAssembly.Memory` objects and `WebAssembly.Table` objects, respectively.

Let `imports` be an initially-empty list of [`external`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L11) values.

For each [`import`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L168)
`i` in `module.imports`:

1. Let `o` be the resultant value of performing
   [`Get`](https://tc39.github.io/ecma262/#sec-get-o-p)(`importObject`, [`i.module_name`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L170)).
1. If `Type(o)` is not Object, throw a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror).
1. Let `v` be the value of performing [`Get`](https://tc39.github.io/ecma262/#sec-get-o-p)(`o`, [`i.item_name`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/ast.ml#L171))
1. If `i` is a function import:
   1. If [`IsCallable(v)`](https://tc39.github.io/ecma262/#sec-iscallable) is `false`,
      throw a `WebAssembly.LinkError`.
   1. If `v` is an [Exported Function Exotic Object](#exported-function-exotic-objects):
      1. (The signature of `v.[[Closure]]` is checked against the import's declared
         [`func_type`](https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#func_type)
         by `Eval.init` below.)
      1. Let `closure` be `v.[[Closure]]`.
   1. Otherwise:
      1. Let `closure` be a new [host function](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L9) value
         of the given signature and the following behavior:
      1. If the signature contains an `i64` (as argument or result), the host
         function immediately throws a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
         when called.
      1. Otherwise, the host function calls `v` with an `undefined` receiver
         and WebAssembly arguments coerced to JavaScript arguments
         via [`ToJSValue`](#tojsvalue). The result is returned by coercing
         via [`ToWebAssemblyValue`](#towebassemblyvalue).
   1. Append `v` to `funcs`.
   1. Append `closure` to `imports`.
1. If `i` is a global import:
   1. [Assert](https://tc39.github.io/ecma262/#assert): the global is immutable
      by MVP validation constraint.
   1. If the `global_type` of `i` is `i64` or `Type(v)` is not Number, throw a `WebAssembly.LinkError`.
   1. Append [`ToWebAssemblyValue`](#towebassemblyvalue)`(v)` to `imports`.
1. If `i` is a memory import:
   1. If `v` is not a [`WebAssembly.Memory` object](#webassemblymemory-objects),
      throw a `WebAssembly.LinkError`.
   1. (The imported `Memory`'s `length` and `maximum` properties are checked against the import's declared
      [`memory_type`](https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#memory_type)
      by `Eval.init` below.)
   1. Append `v` to `memories`.
   1. Append `v.[[Memory]]` to `imports`.
1. Otherwise (`i` is a table import):
   1. If `v` is not a [`WebAssembly.Table` object](#webassemblytable-objects),
      throw a `WebAssembly.LinkError`.
   1. (The imported `Table`'s `length`, `maximum` and `element` properties are checked against the import's declared
      [`table_type`](https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#table_type)
      by `Eval.init` below.)
   1. Append `v` to `tables`.
   1. Append `v.[[Table]]` to `imports`.
   1. For each index `i` of `v.[[Table]]`:
      1. Let `e` be the `i`the element of `v.[[Table]]`.
   1. If `e` is a [`closure`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L7) `c`:
      1. Append the `i`th element of `v.[[Values]]` to `funcs`.

Let `instance` be the result of creating a new
[`instance`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L17)
by calling
[`Eval.init`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/eval.ml#L416)
given `module` and `imports`.
If this terminates with a `Link` error, throw a `WebAssembly.LinkError`; if it causes a trap, throw a `WebAssembly.RuntimeError`; all other exceptions are propagated to the caller.
Among other things, this function performs the following observable steps:

* If, after evaluating the `offset` [initializer expression](Modules.md#initializer-expression)
  of every [Data](Modules.md#data-section) and [Element](Modules.md#elements-section)
  Segment, any of the segments do not fit in their respective Memory or Table, throw a 
  `WebAssembly.LinkError`.

* Apply all Data and Element segments to their respective Memory or Table in the
  order in which they appear in the module. Segments may overlap and, if they do,
  the final value is the last value written in order. Note: there should be no
  errors possible that would cause this operation to fail partway through. After
  this operation completes, elements of `instance` are visible and callable
  through [imported tables](Modules.md#imports), even if `start` fails.

* If a [`start`](Modules.md#module-start-function) is present, it is evaluated.
  Any errors thrown by `start` are propagated to the caller.

The following steps are performed _before_ the `start` function executes:

1. For each table 't' in [`instance.tables`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L17):
   1. If there is no element in `tables` whose `table.[[Table]]` is `t`:
      1. Let `table` be a new `WebAssembly.Table` object with [[Table]] set to `t` and [[Values]] set to a new list of the same length all whose entries are `null`.
      1. Append `table` to `tables`.
   1. Otherwise:
      1. Let `table` be the element in `tables` whose `table.[[Table]]` is `t`
   1. (Note: At most one `WebAssembly.Table` object is created for any table, so the above `table` is unique, even if there are multiple occurrances in the list. Moreover, if the item was an import, the original object will be found.)
   1. For each index `i` of `t`:
      1. Let `c` be the `i`th element of `t`
      1. If `c` is a [`closure`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L7) `c`:
         1. If there is an [Exported Function Exotic Object](#exported-function-exotic-objects) in `funcs` whose `[[Closure]]` equals `c`:
            1. Let `func` be that function object.
         1. (Note: At most one wrapper is created for any closure, so `func` is uniquely determined. Moreover, if the item was an import that is already an [Exported Function Exotic Object](#exported-function-exotic-objects), then the original function object will be found. For imports that are regular JS functions, a new wrapper will be created.)
         1. Otherwise:
            1. Let `func` be an [Exported Function Exotic Object](#exported-function-exotic-objects) created from `c`.
            1. Append `func` to `funcs`.
         1. Set the `i`th element of `table.[[Values]]` to `func`.

(Note: The table and element function objects created by the above steps are only observable for tables that are either imported or exported.)

Let `exports` be a list of (string, JS value) pairs that is mapped from 
each [external](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L24) value `e` in `instance.exports` as follows:

1. If `e` is a [closure](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L12) `c`:
   1. If there is an [Exported Function Exotic Object](#exported-function-exotic-objects) `func` in `funcs` whose `func.[[Closure]]` equals `c`, then return `func`.
   1. (Note: At most one wrapper is created for any closure, so `func` is unique, even if there are multiple occurrances in the list. Moreover, if the item was an import that is already an [Exported Function Exotic Object](#exported-function-exotic-objects), then the original function object will be found. For imports that are regular JS functions, a new wrapper will be created.)
   1. Otherwise:
      1. Let `func` be an [Exported Function Exotic Object](#exported-function-exotic-objects) created from `c`.
      1. Append `func` to `funcs`.
      1. Return `func`.
1. If `e` is a [global](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L15) `v`:
   1. [Assert](https://tc39.github.io/ecma262/#assert): the global is immutable
      by MVP validation constraint.
   1. If `v` is an `i64`, throw a `WebAssembly.LinkError`.
   1. Return [`ToJSValue`](#tojsvalue)`(v)`.
1. If `e` is a [memory](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L14) `m`:
   1. If there is an element `memory` in `memories` whose `memory.[[Memory]]` is `m`, then return `memory`.
   1. (Note: At most one `WebAssembly.Memory` object is created for any memory, so the above `memory` is unique, even if there are multiple occurrances in the list. Moreover, if the item was an import, the original object will be found.)
   1. Otherwise:
      1. Let `memory` be a new `WebAssembly.Memory` object created via [`CreateMemoryObject`](#creatememoryobject) from `m`.
      1. Append `memory` to `memories`.
      1. Return `memory`.
1. Otherwise `e` must be a [table](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L13) `t`:
   1. Assert: There is an element `table` in `tables` whose `table.[[Table]]` is `t`.
   1. Return that `table`.

Let `exportsObject` be a new [frozen](https://tc39.github.io/ecma262/#sec-object.freeze)
plain JS object with [[Prototype]] set to Null and with properties defined
by mapping each export in `exports` to an enumerable, non-writable,
non-configurable data property. Note: the validity and uniqueness checks
performed during [module validation](#webassemblymodule-constructor) ensure
that each property name is valid and no properties are defined twice.

Let `instanceObject` be a new `WebAssembly.Instance` object setting
the internal `[[Instance]]` slot to `instance` and the `[[Exports]]` slot to
`exportsObject`.

Return `instanceObject`.

### `WebAssembly.Instance.prototype [ @@toStringTag ]` Property

The initial value of the [`@@toStringTag`](https://tc39.github.io/ecma262/#sec-well-known-symbols)
property is the String value `"WebAssembly.Instance"`.

This property has the attributes { [[Writable]]: `false`, [[Enumerable]]: `false`, [[Configurable]]: `true` }.

### `WebAssembly.Instance.prototype.exports` property

This is an accessor property whose [[Set]] is Undefined and whose [[Get]]
accessor function performs the following steps:

Let `T` be the `this` value. If `T` is not a `WebAssembly.Instance`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Return `T.[[Exports]]`.

## Exported Function Exotic Objects

A function with [function index](Modules.md#function-index-space) `index`
from an `Instance` `inst` is reflected to JS via a new kind of *Exported
Function* [Exotic Object](https://tc39.github.io/ecma262/#sec-built-in-exotic-object-internal-methods-and-slots).
Like [Bound Function](https://tc39.github.io/ecma262/#sec-bound-function-exotic-objects) Exotic Object,
Exported Functions do not have the normal function internal slots but instead have:

 * [[Closure]] : the [closure](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/instance.ml#L7)
   (`index`, `inst`)

as well as the internal slots required of all builtin functions:

 * [[Prototype]] : [%FunctionPrototype%](https://tc39.github.io/ecma262/#sec-well-known-intrinsic-objects)
 * [[Extensible]] : `true`
 * [[Realm]] : the [current Realm Record](https://tc39.github.io/ecma262/#current-realm)
 * [[ScriptOrModule]] : [`GetActiveScriptOrModule`](https://tc39.github.io/ecma262/#sec-getactivescriptormodule)

Exported Functions also have the following data properties:

* the `length` property is set to the exported function's signature's arity 
* the `name` is set to [`ToString`](https://tc39.github.io/ecma262/#sec-tostring)(`index`)

WebAssembly Exported Functions have a `[[Call]](this, argValues)` method defined as:

1. Let `sig` be the [`function type`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/eval.ml#L106)
   of the function's [[Closure]].
1. If `sig` contains an `i64` (as argument or result), a
   [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
   is thrown each time the [[Call]] method is invoked.
1. Let `args` be an empty list of coerced values.
1. Let `inArity` be the number of arguments and `outArity` be the number of results in `sig`.
1. For all values `v` in `argValues`, in the order of their appearance:
  1. If the length of`args` is less than `inArity`, append [`ToWebAssemblyValue`](#towebassemblyvalue)`(v)` to `args`.
1. While the length of `args` is less than `inArity`, append [`ToWebAssemblyValue`](#towebassemblyvalue)`(undefined)` to `args`.
1. Let `ret` be the result of calling [`Eval.invoke`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/eval.ml#L443)
   passing [[Closure]], and `args`.
1. If `outArity` is 0, return `undefined`.
1. Otherwise, return [`ToJSValue`](#tojsvalue)`(v)`, where `v` is the singular element of `ret`.

`[[Call]](this, argValues)` executes in the [[Realm]] of the callee Exported Function. This corresponds to [the requirements of builtin function objects in JavaScript](https://tc39.github.io/ecma262/#sec-built-in-function-objects).

Exported Functions do not have a [[Construct]] method and thus it is not possible to 
call one with the `new` operator.

## `WebAssembly.Memory` Objects

A `WebAssembly.Memory` object contains a single [linear memory](Semantics.md#linear-memory)
which can be simultaneously referenced by multiple `Instance` objects. Each
`Memory` object has two internal slots:

 * [[Memory]] : a [`Memory.memory`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/memory.mli)
 * [[BufferObject]] : the current `ArrayBuffer` whose [[ArrayBufferByteLength]]
   matches the current byte length of [[Memory]]

### `WebAssembly.Memory` Constructor

The `WebAssembly.Memory` constructor has the signature:

```
new Memory(memoryDescriptor)
```

If the NewTarget is `undefined`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
exception is thrown (i.e., this constructor cannot be called as a function without `new`).

If `Type(memoryDescriptor)` is not Object, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Let `initial` be [`ToNonWrappingUint32`](#tononwrappinguint32)([`Get`](https://tc39.github.io/ecma262/#sec-get-o-p)(`memoryDescriptor`, `"initial"`)).

If [`HasProperty`](https://tc39.github.io/ecma262/#sec-hasproperty)(`"maximum"`),
then let `maximum` be [`ToNonWrappingUint32`](#tononwrappinguint32)([`Get`](https://tc39.github.io/ecma262/#sec-get-o-p)(`memoryDescriptor`, `"maximum"`)).
If `maximum` is smaller than `initial`, then throw a [`RangeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-rangeerror).
Otherwise, let `maximum` be `None`.

Let `memory` be the result of calling 
[`Memory.create`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/memory.ml#L68)
given arguments `initial` and `maximum`. Note that `initial` and `maximum` are
specified in units of WebAssembly pages (64KiB).

Return the result of [`CreateMemoryObject`](#creatememoryobject)(`memory`).

### CreateMemoryObject

Given a [`Memory.memory`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/memory.mli#L1)
`m`, to create a `WebAssembly.Memory`:

Let `buffer` be a new `ArrayBuffer` whose
[[[ArrayBufferData]]](https://tc39.github.io/ecma262/#sec-properties-of-the-arraybuffer-prototype-object)
aliases `m` and whose 
[[[ArrayBufferByteLength]]](https://tc39.github.io/ecma262/#sec-properties-of-the-arraybuffer-prototype-object)
is set to the byte length of `m`.

Any attempts to [`detach`](https://tc39.github.io/ecma262/#sec-detacharraybuffer) `buffer` *other* than
the detachment performed by [`m.grow`](#webassemblymemoryprototypegrow) shall throw a 
[`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)

Return a new `WebAssembly.Memory` instance with [[Memory]] set to `m` and
[[BufferObject]] set to `buffer`.

### `WebAssembly.Memory.prototype [ @@toStringTag ]` Property

The initial value of the [`@@toStringTag`](https://tc39.github.io/ecma262/#sec-well-known-symbols)
property is the String value `"WebAssembly.Memory"`.

This property has the attributes { [[Writable]]: `false`, [[Enumerable]]: `false`, [[Configurable]]: `true` }.

### `WebAssembly.Memory.prototype.grow`

The `grow` method has the signature:

```
grow(delta)
```

Let `M` be the `this` value. If `M` is not a `WebAssembly.Memory`,
a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Let `d` be [`ToNonWrappingUint32`](#tononwrappinguint32)(`delta`).

Let `ret` be the current size of memory in pages (before resizing).

Perform [`Memory.grow`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/memory.mli#L27)
with delta `d`. On failure, a 
[`RangeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-rangeerror)
is thrown.

Perform [`DetachArrayBuffer`](https://tc39.github.io/ecma262/#sec-detacharraybuffer)(`M.[[BufferObject]]`).

Assign to `M.[[BufferObject]]` a new `ArrayBuffer` whose
[[[ArrayBufferData]]](https://tc39.github.io/ecma262/#sec-properties-of-the-arraybuffer-prototype-object)
aliases `M.[[Memory]]` and whose 
[[[ArrayBufferByteLength]]](https://tc39.github.io/ecma262/#sec-properties-of-the-arraybuffer-prototype-object)
is set to the new byte length of `M.[[Memory]]`.

Return `ret` as a Number value.

### `WebAssembly.Memory.prototype.buffer`

This is an accessor property whose [[Set]] is Undefined and whose [[Get]]
accessor function performs the following steps:

If `this` is not a `WebAssembly.Memory`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown. Otherwise return `M.[[BufferObject]]`.

## `WebAssembly.Table` Objects

A `WebAssembly.Table` object contains a single [table](Semantics.md#table)
which can be simultaneously referenced by multiple `Instance` objects. Each
`Table` object has two internal slots:

 * [[Table]] : a [`Table.table`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/table.mli#L1)
 * [[Values]] : an array whose elements are either `null` or [Exported Function Exotic Object](#exported-function-exotic-objects)

### `WebAssembly.Table` Constructor

The `WebAssembly.Table` constructor has the signature:

```
new Table(tableDescriptor)
```

If the NewTarget is `undefined`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
exception is thrown (i.e., this constructor cannot be called as a function without `new`).

If `Type(tableDescriptor)` is not Object, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Let `element` be the result of calling [`Get`](https://tc39.github.io/ecma262/#sec-get-o-p)(`tableDescriptor`, `"element"`).
If `element` is not the string `"anyfunc"`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.
(Note: this check is intended to be relaxed in the
[future :unicorn:][future types] to allow different element types.)

Let `initial` be [`ToNonWrappingUint32`](#tononwrappinguint32)([`Get`](https://tc39.github.io/ecma262/#sec-get-o-p)(`tableDescriptor`, `"initial"`)).

If [`HasProperty`](https://tc39.github.io/ecma262/#sec-hasproperty)(`"maximum"`),
then let `maximum` be [`ToNonWrappingUint32`](#tononwrappinguint32)([`Get`](https://tc39.github.io/ecma262/#sec-get-o-p)(`tableDescriptor`, `"maximum"`)). Otherwise, let `maximum` be None.

If `maximum` is not None and is smaller than `initial`, then throw a [`RangeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-rangeerror).

Let `table` be the result of calling 
[`Table.create`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/table.ml#L68)
given arguments `AnyFuncType`, `initial` and `maximum`.

Let `values` be a new empty array of `initial` elements, all with value
`null`.

Return a new `WebAssemby.Table` instance with [[Table]] set to `table` and
[[Values]] set to `values`.

### `WebAssembly.Table.prototype.length`

This is an accessor property whose [[Set]] is Undefined and whose [[Get]]
accessor function performs the following steps:

Let `T` be the `this` value. If `T` is not a `WebAssembly.Table`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Return `T.[[Values]].length`.

### `WebAssembly.Table.prototype.grow`

The `grow` method has the signature:

```
grow(delta)
```

Let `T` be the `this` value. If `T` is not a `WebAssembly.Table`,
a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Let `d` be [`ToNonWrappingUint32`](#tononwrappinguint32)(`delta`).

Let `ret` be the current length of the table (before resizing).

Perform [`Table.grow`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/table.ml#L40),
with delta `d`. On failure, a
[`RangeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-rangeerror)
is thrown.

Return `ret` as a Number value.

### `WebAssembly.Table.prototype.get`

This method has the following signature

```
get(index)
```

Let `T` be the `this` value. If `T` is not a `WebAssembly.Table`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

Let `i` be the result of [`ToNonWrappingUint32`](#tononwrappinguint32)(`index`).

If `i` is greater or equal than the length of `T.[[Values]]`, a [`RangeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-rangeerror) is thrown.

Return `T.[[Values]][i]`.

### `WebAssembly.Table.prototype.set`

This method has the following signature

```
set(index, value)
```

Let `T` be the `this` value. If `T` is not a `WebAssembly.Table`, a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror)
is thrown.

If `value` is not an [Exported Function Exotic Object](#exported-function-exotic-objects)
or `null`, throw a [`TypeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror).

Let `i` be the result of [`ToNonWrappingUint32`](#tononwrappinguint32)(`index`).

If `i` is greater or equal than the length of `T.[[Values]]`, a [`RangeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-rangeerror) is thrown.

If `value` is `null`, let `elem` be `Uninitialized`;
otherwise, let `elem` be `value.[[Closure]]`.

Set `T.[[Table]][i]` to `elem`.

Set `T.[[Values]][i]` to `value`.

Return `undefined`.

### `WebAssembly.Table.prototype [ @@toStringTag ]` Property

The initial value of the [`@@toStringTag`](https://tc39.github.io/ecma262/#sec-well-known-symbols)
property is the String value `"WebAssembly.Table"`.

This property has the attributes { [[Writable]]: `false`, [[Enumerable]]: `false`, [[Configurable]]: `true` }.

## ToJSValue

To coerce a WebAssembly [`value`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/values.ml#L9)
to a JavaScript value:

Assert: the WebAssembly value's type is not `i64`.

1. given a WebAssembly `i32` is interpreted as a signed integer, converted (losslessly) to an
  IEEE754 double and then returned as a JavaScript Number
1. given a WebAssembly `f32` (single-precision IEEE754), convert (losslessly) to
   a IEEE754 double, [possibly canonicalize NaN](https://tc39.github.io/ecma262/#sec-setvalueinbuffer),
   and return as a JavaScript Number
1. given a WebAssembly `f64`, [possibly canonicalize NaN](https://tc39.github.io/ecma262/#sec-setvalueinbuffer)
   and return as a JavaScript Number

If the WebAssembly value is optional, then given `None`, return JavaScript value
`undefined`.

## ToWebAssemblyValue

To coerce a JavaScript value to a given WebAssembly [`value type`](https://github.com/WebAssembly/spec/blob/master/interpreter/spec/types.ml#L3),

Assert: the target value type is not `i64`.

1. coerce to `i32` via [`ToInt32(v)`](https://tc39.github.io/ecma262/#sec-toint32)
1. coerce to `f32` by first applying [`ToNumber(v)`](https://tc39.github.io/ecma262/#sec-tonumber)
   and then converting the resulting IEEE754 64-bit double to a 32-bit float using `roundTiesToEven`
1. coerce to `f64` via [`ToNumber(v)`](https://tc39.github.io/ecma262/#sec-tonumber)

If the value type is optional, then given `None`, the JavaScript value is
ignored.

## ToNonWrappingUint32

To convert a JavaScript value `v` to an unsigned integer in the range [0, `UINT32_MAX`]:

Let `i` be [`ToInteger`](https://tc39.github.io/ecma262/#sec-tointeger)(`v`).

If `i` is negative or greater than `UINT32_MAX`, 
[`RangeError`](https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-rangeerror)
is thrown.

Return `i`.

## Sample API Usage

Given `demo.was` (encoded to `demo.wasm`):

```lisp
(module
    (import "js" "import1" (func $i1))
    (import "js" "import2" (func $i2))
    (func $main (call $i1))
    (start $main)
    (func (export "f") (call $i2))
)
```

and the following JavaScript, run in a browser:

```javascript
var importObj = {js: {
    import1: () => console.log("hello,"),
    import2: () => console.log("world!")
}};
fetch('demo.wasm').then(response =>
    response.arrayBuffer()
).then(buffer =>
    WebAssembly.instantiate(buffer, importObj)
).then(({module, instance}) =>
    instance.exports.f()
);
```

[future general]: FutureFeatures.md
[future types]: FutureFeatures.md#more-table-operators-and-types